This invention relates to closures or clasps for securing conductive grounding straps about the wrist or limb.
Static electricity causes problems for electronics manufacturers and those in other industries, particularly with the advent of integrated circuits and other microelectronic components. Components such as integrated circuits, for instance, may be disabled or destroyed by over-voltages or power density resulting from static electricity. Certain junctions in such circuits can be destroyed by as little as a 50-volt potential, which radically changes the doping structure in their lattices. Power densities resulting from excessive potential and imperfections in circuit layout or structure can vaporize or radically alter the silicon substrate and thus impair or destroy a circuit's performance. Yet a person walking on a carpet on a dry day can accumulate as much as 30,000 volts of potential, and he can triboelectrically generate thousands of volts by simply changing his position in his chair or handling a styrofoam cup.
Such a person can inadvertently discharge such static electric potential into the circuit or component by touching it and causing over-voltage or excessive power density. Additionally, the potential in such a person's body can induce a charge in a circuit that can later cause over-voltage or excessive power density when the circuit is subsequently grounded.
More and more frequently, therefore, those in industries in which integrated circuits and other microelectronic components are handled or assembled are taking measures to limit the failure rate of those circuits and components by attempting to keep them as well as their environment at zero electrical potential. Such measures include providing workers and work stations with anti-static carpet, conductive or dissipative grounded desk top work surfaces, hot air ion generators which emit ions to neutralize static charges, and grounding straps to keep workers at zero potential. The term "conductive" herein, and according to its customary usage in the art, means an electrical resistance of between zero and 10.sup.5 ohms. Similary, "dissipative" means a resistance of between 10.sup.5 and 10.sup.9 ohms, "anti-static" means a resistance of between 10.sup.9 and 10.sup.14 ohms, and "insulative" means a resistance of more than 10.sup.14 ohms.
A grounding strap must have several features in order to perform its grounding function effectively. First, it must ensure that the wearer's skin is electrically connected to ground. This connection is typically accomplished by a conductive surface on the inside of the strap contacting the skin. The conductive surface is electrically connected to a grounding cord which leads from the strap to a grounded electrical connection. If the electrical contacting means on the inside of the strap becomes dirty or fouled by oil, perspiration or hair, the strap may lose its effectiveness. It is therefore important to use a conductive material on the inner surface of the strap that does not easily become dirty or fouled.
Second, comfort is a premium consideration, because if the strap is uncomfortable, the wearer will be tempted to remove it and can thereby cause damage to electrical components on which he is working. A strap that is easily stretchable, that breathes, that is attractive and that poses minimum inconvenience to the wearer is therefore highly desired.
The situations in which grounding wrist straps are used heightens the importance of their being comfortable so that they are continuously worn and maintain continuous electrical contact with the skin. A person working on microelectronic components or integrated circuits may be completely unaware that he has accumulated minor static electrical charges and he may therefore unwittingly be in a position to disable circuits on which he is working or which he is handling. If his strap is loose or he has removed it, he may be unaware that electrical discharges transmitted from his fingers are disabling these circuits. (A typical person cannot sense a static electrical discharge of less than approximately 3,500 volts.) No one may discover that the circuits have been disabled or damaged until hours, days or weeks later, when the circuits have been placed in components or devices which fail in the field. Removal and repair or replacement of these circuits once in the field is far costlier than avoiding potential failure while the wearer is handling the circuits. Thus, the wearer's employer typically must depend upon the effectiveness of the wrist strap to maintain a lower failure rate of such electronic circuits and components, by maintaining continuous electrical contact with the wearer's wrist and by providing him minimum temptation to remove the strap from his wrist.
Several approaches have been followed in an attempt to address these considerations. An extensible metal band similar to a Speidel.RTM. watchband, for instance, is disclosed in U.S. Pat. No. 4,373,175 issued Feb. 8, 1983 to Mykkanen. A strap made of an outer and inner conductive polyolefin layer which sandwich an intermediate nylon scrim layer is disclosed in U.S. Pat. No. 3,857,397 issued Dec. 31, 1984 to Brosseau. That patent shows the use of hook and loop fastening material to hold the strap adjustably on the wrist.
A knitted stretchable fabric strap containing stainless steel fibers is disclosed in U.S. Pat. No. 4,398,277 issued Aug. 9, 1983 to Christiansen and Westberg. The knitted fabric material of the strap is perhaps cooler and lighter than the metal or plastic straps mentioned above, but a plastic and metal fitting permanently closes the strap into a loop of predetermined size. The wearer thus cannot adjust the strap to improve electrical contact or to reduce constriction about the wrist.
My U.S. Pat. No. 4,577,256 issued Mar. 18, 1986 entitled WOVEN STRETCHABLE GROUNDING STRAP and mentioned above discloses a fabric strap that has an adjustable closure or clasp. That patent is incorporated herein by reference. The clasp avoids the need to manufacture two or more models of the strap for different size wrists. It also makes the strap markedly more comfortable and effective for all sizes of wrists. The closure reduces the wearer's temptation to remove the strap and thus reduces the chance that circuits and components will be damaged by the wearer's static charges.
My adjustable clasp mentioned above includes a fastener for connection to a grounding cord. The fastener passes through the clasp's body and is electrically and physically connected to a metallic plate. The metallic plate contacts the wearer's wrist or limb as well as conductive yarns on the inner surface of the conductive elastic material of the strap.
Because the fastener passes from the outer to the inner surface of the clasp body, it prevents the possibility of the conductive elastic material being threaded through the strap from one end to the other. The loose end of the material to be secured by the clasp thus enters the clasp body from the end of the clasp opposite the fastener. The material is secured by a pivotally mounted gate and then exits the clasp on the clasp's top surface. The conductive yarns of the material as a result are exposed. To reduce the possibility of accidental contact with dangerous electrical potential, the material extending beyond the top of the clasp body is typically cut after the strap is adjusted to fit the wearer's wrist. Once cut, the strap obviously cannot be adjusted to accommodate larger wrist sizes.